The invention relates generally to wavelength stabilization of broadband sources and more specifically to a method and apparatus for stabilizing a characteristic wavelength of a broadband source using an optical power divider and a stable reference optical source.
Stable broadband sources are useful in a variety of applications including, for example, telecommunications test equipment, optical tomography systems, optical distance measurement systems, and interferometric fiber optic gyroscopic (IFOG) navigation equipment. Various methods to stabilize a broadband source have been developed. For example, one method employs an unbalanced interferometer stabilized to a Helium-Neon laser reference source. The broadband source to be controlled, however, must have a coherence length at least as long as the difference in the optical paths of the interferometer. Unfortunately, reducing the difference in the optical paths typically reduces the wavelength sensitivity of the method. Thus, this method is highly dependent on the degree of residual coherence in the broadband source.
Another method developed for use with IFOG navigation equipment employs a directional coupler as a wavelength reference. Optical energy from a broadband source is injected into the directional coupler. The coupler separates the optical energy into two output ports. The optical energy is divided between the two output ports according to wavelength. The optical power exiting the two outputs of the directional coupler is balanced, thereby matching the center wavelength of the broadband source to the characteristic center wavelength of the directional coupler, which serves as the wavelength reference for the broadband source. One disadvantage of this technique is that temperature variations change the characteristic center wavelength of the directional coupler. Also, the two photodetectors used to measure the power output from the two output arms of the coupler must have closely matched responsivities. Any systematic influence which affects the responsivity match of the photodetectors and/or changes the center wavelength of the directional coupler results in an unwanted shift in the center wavelength of the broadband source. For precise wavelength control, the directional coupler and photodetectors are typically housed in a temperature-stabilized environment, which can be prohibitive for applications in which space is limited. Additionally, the central wavelength of the directional coupler is sensitive to gamma and neutron radiation exposure, making this method impractical for strategic guidance applications.
A need exists for a method to stabilize broadband sources that is substantially insensitive to environmental factors and not limited by coherence length.
The invention relates to a method and system for wavelength stabilization of a broadband optical source. The method and system are based on utilizing an optical power divider to generate two optical signals for each of the broadband source and a reference wavelength source. The difference in the power ratio of the two optical signals derived from the broadband source and the power ratio of the two optical signals derived from the reference wavelength source is determined. Because the power ratios are similarly affected by component aging and changes in environmental factors such as temperature and incident radiation, the difference in the power ratios can be used to adjust the wavelength of the broadband source so that its center wavelength is stabilized to the center wavelength of the reference source.
In one aspect, the invention relates to a method for wavelength stabilization of a broadband optical source having a center wavelength. The method includes generating a first split broadband optical signal and a second split broadband optical signal in response to a wavelength transfer characteristic and the broadband optical signal. The method also includes generating a first split optical reference signal and a second split optical reference signal in response to the wavelength transfer characteristic and a reference optical signal having a reference wavelength. The method further includes determining a broadband power ratio of the first and second split optical signals, determining a reference power ratio of the first and second split optical reference signals, and determining a difference between the center wavelength and the reference wavelength in response to the power ratios. In one embodiment the method also includes adjusting the center wavelength of the broadband optical signal in response to the difference of the center wavelength and the reference wavelength.
In another aspect, the invention relates to a system for generating a wavelength stabilized optical signal including a broadband optical source, a wavelength reference module and an optical power divider. The broadband optical source is configured to provide a broadband optical signal having a broadband center wavelength and the wavelength reference module is configured to provide a reference optical signal having a reference center wavelength. The optical power divider has a divider input port in optical communication with the broadband optical source and the wavelength reference module. The optical power divider also has a first divider output port and a second divider output port. The optical divider is configured to provide a first split broadband optical signal and a second split broadband optical signal at the first and second divider output ports, respectively, in response to the broadband optical signal. The optical power divider is also configured to provide a first split reference optical signal and a second split reference optical signal at the first and second divider output ports, respectively, in response to the reference optical signal. The power ratio of the first and second split optical signals is responsive to the broadband center wavelength and the power ratio of the first and second split reference optical signals is responsive to the reference center wavelength.
In one embodiment the system also includes a first detector in optical communication with the first divider output port and a second detector in optical communication with the second divider output port. In a further embodiment, the system also includes a wavelength control module in communication with the first and second detectors. The wavelength control module determines the power ratio of the first and second split broadband optical signals and also determines the power ratio of the first and second split reference optical signals. A broadband source control signal is generated by the wavelength control module in response to the power ratios.
In one embodiment the system also includes an optical combiner in optical communication with the broadband optical source, the wavelength reference module and the optical power divider. In another embodiment, the system includes a broadband optical modulator in optical communication with the broadband optical source and the optical power divider. In another embodiment, the system includes a reference optical modulator in optical communication with the wavelength reference module and the optical power divider. In another embodiment, the system includes an optical modulator in optical communication with the broadband optical source, the wavelength reference module, and the optical power divider.